Hydraulic systems are utilized in many forms of construction equipment such as hydraulic excavators, backhoe loaders, and end loaders. The equipment is usually mobile having either wheels or track and includes a number of hydraulically actuated devices such as hydraulic cylinders and motors. In most cases, hydraulic systems are controlled by a valve arrangement in which a hydraulic pump provides pressurized fluid to a plurality of valves each associated with a hydraulic cylinder or motor. As an operator manipulates control levers located in the operator's compartment, hydraulic valves are controllably opened and closed such that pressurized fluid is controllably directed to the desired cylinder or motor.
Known systems typically utilize a plurality of open-center control valves to controllably actuate the various hydraulic actuators on the machine. The open-center control valves permit the system to exhibit a variable response which is dependent upon the load on the actuator. Accordingly, with an open-center valve arrangement, the operator typically can control a combination of both the velocity and the force, or pressure, of the hydraulic cylinder. However, control, or modulation, of cylinder velocity with open center control valves is not consistent. This lack of consistent control manifests itself in the form of variable deadband and gain in actual operation.
One way to achieve constant and more predictable results with respect to velocity modulation is the utilization of a closed-center load sensing system. In a closed-center load sensing system the valve flow is independent of load pressure. Although this results in consistent velocity modulation it does not provide the ability to modulate or limit cylinder force. In such a system, the operator loses the sense or "feel" for the load.
It is desirable to have drive systems which can exhibit some characteristics of both open-center and closed-center systems in the form of velocity modulation control and force modulation control. The present invention is directed to overcoming one or more of the foregoing problems associated with known hydraulic control systems.